Please refer to FIG. 1 that is a perspective view of a conventional filter screen structure 10 formed from a corrugated filter screen 11 having a plurality of air vents 12 evenly distributed thereon, so that air may smoothly flow through the filter screen 11. The conventional filter screen structure 10 may be generally divided into three types. In the first type, the corrugated filter screen 11 has a fixed corrugation height H and a fixed corrugation pitch P, as shown in FIG. 2A. In the second type, the corrugated filter screen 11 has a fixed corrugation pitch P but different corrugation heights H, H′, as shown in FIG. 2B. In the third type, the corrugated filter screen 11 has a fixed corrugation height H but different corrugation pitches P, P′, as shown in FIG. 2C. FIGS. 2A, 2B, and 2C are side views of the above-mentioned first, second, and third types of the conventional filter screen structure 10, respectively. As can be seen from FIG. 2A, with the fixed corrugation height H and pitch P of the first type of filter screen structure 10, the speed of air passing different areas of the filter screen 11 under a fixed pressure is always the same. In FIG. 2B, with the different corrugation heights H, H′ of the second type of filter screen structure 10, the filter screen 11 has a high impedance at an area with the larger corrugation height H, and a low impedance at an area with the smaller corrugation height H′. Therefore, under a fixed pressure, the speed of air flown through the area of the filter screen 11 having the higher corrugations is smaller than that of the air flown through the area having lower corrugations. In FIG. 2C, with the different corrugation pitches P, P′ of the third type of filter screen structure 10, the filter screen 11 has a high impedance at an area with the smaller corrugation pitch P, and a low impedance at an area with the larger corrugation pitch P′. Therefore, under a fixed pressure, the speed of air flown through the area of the filter screen 11 having the smaller corrugations pitch P is smaller than that of the air flown through the area having the larger corrugation pitch P′.
A main purpose of the above second and third types of the conventional filter screen structure 10 is to produce different air flow rates on the filter screen 11 under a fixed pressure, so as to achieve the object of regulating the volume of air passing through the filter screen 11.
The filter screen 11 is provided with a plurality of air vents 12 evenly distributed thereon for the air to smoothly flow through the filter screen 11. It is observed that the speed of air flown through the filter screen 11 is in direct ratio to a pressure difference between a top and a bottom surface of the filter screen 11. That is, when it is desired for an increased air volume to pass through the filter screen 11, there must be a larger pressure difference between the top and the bottom surface of the filter screen 11. On the other hand, when it is desired for a decreased air volume to pass through the filter screen 11, there must be a smaller pressure difference between the top and the bottom surface of the filter screen 11. With the pressure difference between the top and the bottom surface of the filter screen 11, the airflow may overcome the impedance of the filter screen 11 to pass through the latter.
In the conventional filter screen structure 10, it is necessary to change the corrugation pitch P and the corrugation height H of the filter screen 11 to achieve the purpose of regulating the air flow, as the above-described second and third types of the filter screen structure 10. However, the different corrugation heights and pitches increase difficulties in fabricating and mass-producing the filter screen 11.